The present invention relates to a method for replacing a first cable provided with at least one electrical conductor by a second cable provided with at least one electrical conductor between a first predetermined point and a second predetermined point of the first cable, comprising the following steps:
a. providing the second cable with a first end there of in the vicinity of the first point, the first end being provided with a first series impedance means switchable between states of high and low impedance value; PA1 b. providing the second cable with a second end thereof in the vicinity of the second point, the second end being provided with a second series impedance means switchable between states of high and low impedance value; PA1 c. electrically connecting the at least one conductor of the second cable at the first end to a predetermined conductor of the first cable at the first point while keeping the first impedance means in a state of high impedance and electrically connecting the at least one conductor of the second cable at the second end to the predetermined conductor of the first cable at the second point while keeping the second impedance means in a state of high impedance; PA1 d. substantially simultaneously switching the first series impedance means to a state of low impedance and the second series impedance means to a state of low impedance value; PA1 e. cutting through the first cable between the first point and the second point at a location near the first point and another location near the second point. PA1 providing third series impedance means and fourth series impedance means in the first cable between the first point and the second point, which third impedance means are provided in the vicinity of the first point and which fourth impedance means are provided in the vicinity of the second point, both the third and fourth series impedance means being impedance inducing means allowing induction of an impedance within a range from a low impedance value to a high impedance value without cutting through the first cable; PA1 prior to step e both the third and the fourth series impedance means are kept in their states of low impedance value; PA1 prior to step e, but after the first and second series impedance means have been switched to their states of low impedance values, both the third and the fourth series impedance means are switched to their states of high impedance value.
Such a method is known from Electronics & Communications in Japan, part II, Electronics, part 71, Nr. 7, part 02, Jul. 1, 1988, pages 84-93, Naohisa Komatsu et al.: "Technology of cable transfer system for error-free digital transmission".
The problem which is solved by the present invention will be explained with reference to FIGS. 1a to 1e. When work is carried out on operational telecommunications cables, existing cables frequently have to be replaced by new cables. The conventional method for this is substantially to remove the existing cable except for a few connecting pieces and then to fit a new cable between these connecting pieces. The problem with this is that active telecommunications connections are interrupted as a result. This problem has, in principle, already been solved before by a method which is shown diagrammatically in FIGS. 1a to 1e.
FIG. 1a shows a telecommunications cable 1 between two communication points A and B, a section of which cable between points 1a and 1b has to be replaced by a new cable 2. FIG. 1a shows cable 2, which has two cable ends 2a, 2b, loose alongside cable 1.
FIG. 1b shows that in a first method step cable end 2a is connected to point 1a. The cables 1, 2 are shown diagrammatically. However, it must be understood that the cables can each contain tens, if not hundreds, of cores or core pairs. An engineer has to expose all of these cores and will therefore be working for some time to connect all cores of cables 1 and 2 with one another at point 1a. During this entire time cable end 2b is still not connected to cable 1.
Once the engineer has finished, he moves to point 1b and there connects cable end 2b to cable 1 following the same procedure as at point 1a. This situation is shown in FIG. 1c, from which it can be seen that points 1a and 1b are connected via two parallel communication cables 1 and 2.
FIG. 1d shows that the engineer then cuts through cable 1 at a point which is close to point 1b and is located between points 1a and 1b, so that only the communication link via cable 2 remains intact. He then moves to point 1a in order there also to cut through cable 1 at a point between points 1a and 1b, as is shown in FIG. 1e. The latter figure shows that a cut-through section 1' of cable 1 remains between points 1a and 1b. Section 1' can then be removed as desired.
It will be clear that with the method illustrated in FIGS. 1a to 1e the communication link between points A and B is never broken.
However, with the method shown in FIGS. 1a to 1e problems occur at two points in time with respect to, in particular, broadband signals, which are transmitted at high speed, for example at more than 1 MB/s, between points A and B. These points in time are shown in FIGS. 1b and 1d. It can be seen from FIG. 1b that cable 2 is connected by one or more cores to cable 1 at point 1a, whilst cable end 2b is still open. Especially in the case of broadband signals which are transmitted via cores of cable 1 which have already been connected to corresponding cores of cable 2, undesirable reflections then occur in the cores of cable 2. This can give rise to dips in the transmission. In practice it is found that this can cause interference in modems. The consequence can even be communication failure. These problems also arise in the situation according to FIG. 1d, although it is then not cable 2 which produces the reflections but the section of cable 1 between points 1a and 1b which at that point in time has been cut through on one side only.
In practice, points 1a and 1b are often at least a few tens of metres apart. It would already be possible appreciably to shorten the duration of the undesirable situation by having two engineers working simultaneously at points 1a and 1b, who are able to communicate with one another so that they work as far as possible on the same core pairs at the same time. By this means the undesirable situation per core pair would be limited appreciably. However, even then some delay between making and breaking the connections at points 1a and 1b is still unavoidable.
The prior art disclosed by Komatsu et al. referred to above discloses a method and devices for providing series impedances at those locations of the cable to be replaced that need to be cut through after the latter cable is replaced by a new cable. The known method is carried out by devices comprising switches that can be controlled remotely. A first switch is arranged to bypass a first location in the cable to be replaced which first location needs to be cut through. A second switch is arranged to bypass a second location in the cable to be replaced which second location needs to be cut through. The devices used comprise two further switches and a resistor thus forming a pair transfer circuit, which is also to be connected to the ends of the new cable. Any of the conductors of the old cable to be replaced and the new cable need to be connected to pair transfer circuits of this type. Although the method and the device described by Komatsu et al. overcome the problems related to the method illustrated with reference to FIGS. 1a to 1e, they are still complex and there is a need for simplification.